Detection of PBDE effects on mRNA expression in chicken (Gallus domesticus) neuronal cells using real-time RT-PCR and a new differential display method

Thyroid disrupting effects and the developmental toxicity of hexafluoropropylene oxide oligomer acids in zebrafish during early development

As perfluorooctanoic acid (PFOA) alternatives, hexafluoropropylene oxide dimeric acid (HFPO-DA) and hexafluoropropylene oxide trimeric acid (HFPO-TA) have been increasingly used and caused considerable water pollution. However, their toxicities to aquatic organisms are still not well known. Therefore, in this study, zebrafish embryos were exposed to PFOA (0, 1.5, 3 and 6 mg/L), HFPO-DA (0, 3, 6 and 12 mg/L) and HFPO-TA (0, 1, 2 and 4 mg/L) to comparatively investigate their thyroid disrupting effects and the developmental toxicity. Results demonstrated that waterborne exposure to PFOA and its two alternatives decreased T4 contents, the heart rate and swirl-escape rate of zebrafish embryos/larvae. The transcription levels of genes related to thyroid hormone regulation (crh), biosynthesis (tpo and tg), function (trα and trβ), transport (transthyretin, ttr), and metabolism (dio1, dio2 and ugt1ab), were differently altered after the exposures, which induced the thyroid disrupting effects and decreased the heart rate. In addition, the transcription levels of some genes related to the nervous system development were also significantly affected, which was associated with the thyroid disrupting effects and consequently affected the locomotor activity of zebrafish. Therefore, HFPO-DA and HFPO-TA could not be safe alternatives to PFOA. Further studies to uncover the underlying mechanisms of these adverse effects are warranted.

Ecotoxicology of persistent organic pollutants in birds

Considering the explosive growth of the list of persistent organic pollutants (POPs), the scientific community is combatting increasing challenges to protect humans and wildlife from the potentially negative consequences of POPs. Herein, we characterize the main aspects and progress in the ecotoxicology of POPs in avian species since 2000. The majority of previous efforts has revealed the global occurrence of high levels of various POPs in birds. Laboratory research and epidemiological studies imply that POPs exert a broad-spectrum of side-effects on birds by interfering with their endocrine, immune and neural system, reproduction, and development, and growth. However, inconsistent results suggest that the potential effects of POP exposure on the physiological parameters in birds are multifactorial, involving a multitude of biological processes, species-specific differences, gender, age and types of compounds. Great progress has been achieved in identifying the species-specific sensitivity to dioxin-like compounds, which is attributed to different amino acid residues in the ligand-binding domain of the aryl hydrocarbon receptor. Besides the conventional concentration additivity, several studies have suggested that different classes of POPs possibly act synergistically or antagonistically based on their concentration. However, ecotoxicology information is still recorded in a scattered and inadequate manner, including lack of enough avian species, limited number of POPs investigated, and insufficient geographical representation, and thus our understanding of the effects of POPs on birds remains rudimentary, although mechanistic understanding of their mode of action is progressing. Particularly, research on what happens to wild bird populations and their ecosystems under POP stress is still unavailable. Thus, our aim is to predict and trace the effects POPs at different biological organization levels, especially from the molecular, cellular and individual levels to the population, community and ecosystem levels because of the limited and scattered information, as mentioned above.

The synthetic progestin norethindrone causes thyroid endocrine disruption in adult zebrafish

Synthetic progestin norethindrone (NET) widely used in oral contraceptives, hormonal therapy and livestock farming has been detected in various aquatic ecosystems. Recent studies have shown that NET can cause thyroid endocrine disruption in amphibians. However, studies are still lacking on thyroid axis of fish. In the present study, we investigated thyroid hormone levels (T3 and T4) and transcriptional patterns of 15 genes of the hypothalamic-pituitary-thyroid axis (HPT axis) in adult zebrafish that were exposed to solvent control and three measured concentrations of NET (7, 84 and 810 ng/L) for 90 days. The results indicated that NET significantly lowered T3 and T4 levels in both female and male zebrafish. Transcriptional expression profiles of some of the HPT-axis related genes were disrupted. Specifically, the expression levels of tshb and pax8 have increased significantly while dio2 and ugt1ab have decreased in females. In male, however, tshb expression levels were increased while ttr, ugt1ab, thra and thrb were decreased. The overall results demonstrate that NET disrupts thyroid endocrine system by interfering at multiple sites along HPT axis in adult zebrafish.

Changes in thyroid hormone levels and related gene expressions in embryo-larval zebrafish exposed to binary combinations of bifenthrin and acetochlor

Bifenthrin (BF) and acetochlor (AT) are widely used as an insecticide and herbicide, respectively, which are introduced to the aquatic environment as a natural result. Although the thyroid active substances may coexist in the environment, their joint effects on fish have not been identified. We examined the joint toxicity of BF and AT in zebrafish (Danio rerio) in this study. An acute lethal toxicity test indicated that the median lethal concentration (LC₅₀) values of BF and AT under 96 h treatment were 0.40 and 4.56 µmol L^-1, respectively. The binary mixture of BF + AT displayed an antagonistic effect on the acute lethal toxicity. After 14 days post fertilization (dpf) with exposure to individual pesticides at sub-lethal concentrations of, no effects were observed on the catalase (CAT) and peroxidase (POD) activities, while the binary mixtures (except for the 7.2 × 10^-3 µmol L^-1 BF + 1.2 × 10^-2 µmol L^-1 AT exposure group) significantly induced the CAT activity. The superoxide dismutase (SOD) activity and triiodothyronine (T3) level were significantly increased in all exposure groups. The thyroxine (T4) level remained unchanged after exposure to individual pesticides, but significantly increased in the 7.2 × 10^-3 µmol L^-1 BF + 1.2 × 10^-2 µmol L^-1 AT group. The expressions of the genes Dio2, TRa, TSHβ and CRH in the thyroid hormone (TH) axis were significantly up-regulated in the 7.2 × 10^-3 µmol L^-1 BF + 0.4 × 10^-2 µmol L^-1 AT group. Our data indicated that the binary mixture of BF + AT significantly altered the antioxidant enzyme activities and gene expressions in the hypothalamic-pituitary-thyroid (HPT) axis and changed the TH levels.

Tropism of Newcastle disease virus strains for chicken neurons, astrocytes, oligodendrocytes, and microglia

BACKGROUND

Newcastle disease (ND), which is caused by infections of poultry species with virulent strains of Avian orthoavulavirus-1, also known as avian paramyxovirus 1 (APMV-1), and formerly known as Newcastle disease virus (NDV), may cause neurological signs and encephalitis. Neurological signs are often the only clinical signs observed in birds infected with neurotropic strains of NDV. Experimental infections have shown that the replication of virulent NDV (vNDV) strains is in the brain parenchyma and is possibly confined to neurons and ependymal cells. However, little information is available on the ability of vNDV strains to infect subset of glial cells (astrocytes, oligodendrocytes, and microglia). The objective of this study was to evaluate the ability of NDV strains of different levels of virulence to infect a subset of glial cells both in vitro and in vivo. Thus, neurons, astrocytes and oligodendrocytes from the brains of day-old White Leghorn chickens were harvested, cultured, and infected with both non-virulent (LaSota) and virulent, neurotropic (TxGB) NDV strains. To confirm these findings in vivo, the tropism of three vNDV strains with varying pathotypes (SA60 [viscerotropic], TxGB [neurotropic], and Tx450 [mesogenic]) was assessed in archived formalin-fixed material from day-old chicks inoculated intracerebrally.

RESULTS

Double immunofluorescence for NDV nucleoprotein and cellular markers showed that both strains infected at least 20% of each of the cell types (neurons, astrocytes, and oligodendrocytes). At 24 h post-inoculation, TxGB replicated significantly more than LaSota. Double immunofluorescence (DIFA) with markers for neurons, astrocytes, microglia, and NDV nucleoprotein detected the three strains in all three cell types at similar levels.

CONCLUSION

These data indicate that similar to other paramyxoviruses, neurons and glial cells (astrocytes, oligodendrocytes, and microglia) are susceptible to vNDV infection, and suggest that factors other than cellular tropism are likely the major determinant of the neurotropic phenotype.

Expression of thyroid hormone regulator genes in the yolk sac membrane of the developing chicken embryo

Thyroid hormones (THs) are essential for the correct development of nearly every structure in the body from the very early stages of development, yet the embryonic thyroid gland is not functional at these stages. To clarify the roles of the egg yolk as a source of THs, the TH content in the yolk and the expression of TH regulator genes in the yolk sac membrane were evaluated throughout the 21-day incubation period of chicken embryos. The yolk TH content (22.3 ng triiodothyronine and 654.7 ng thyroxine per total yolk on day 4 of incubation) decreased almost linearly along with development. Real-time PCR revealed gene expression of transthyretin, a principal TH distributor in the chicken, and of a TH-inactivating iodothyronine deiodinase (DIO3), until the second week of incubation when the embryonic pituitary-thyroid axis is generally thought to start functioning. The TH-activating deiodinase (DIO2) and transmembrane transporter of thyroxine (SLCO1C1) genes were expressed in the last week of incubation, which coincided with a marked increase of circulating thyroxine and a reduction in the yolk sac weight. DIO1, which can remove iodine from inactive THs, was expressed throughout the incubation period. It is assumed that the chicken yolk sac inactivates THs contained abundantly in the yolk and supplies the hormones to the developing embryo in appropriate concentrations until the second week of incubation, while THs may be activated in the yolk sac membrane in the last week of incubation. Additionally, the yolk sac could serve as a source of iodine for the embryo.

Developmental toxicity and thyroid hormone-disrupting effects of 2,4-dichloro-6-nitrophenol in Chinese rare minnow (Gobiocypris rarus)

In the present study, to evaluate embryonic toxicity and the thyroid-disrupting effects of 2,4-dichloro-6-nitrophenol (DCNP), embryos and adults of Chinese rare minnow (Gobiocypris rarus) were exposed to 2, 20, and 200μg/L DCNP. In the embryo-larval assay, increased percentages of mortality and occurrence of malformations, decreased percentage of hatching, and decreased body length and body weight were observed after DCNP treatment. Moreover, the whole-body T3 levels were significantly increased at 20 and 200μg/L treatments, whereas the T4 levels were markedly decreased significantly (p<0.05) for all DCNP concentrations. In the adult fish assay, plasma T3 levels were significantly increased whereas plasma T4 levels were significantly reduced in the fish treated with 20 and 200μg/L (p<0.05). In addition, DCNP exposure significantly changed the transcription levels of thyroid system related genes, including dio1, dio2, me, nis, tr, and ttr. The increased responsiveness of thyroid hormone and mRNA expression levels of thyroid system related genes suggested that DCNP could disrupt the thyroid hormone synthesis and transport pathways. Therefore, our findings provide new insights of DCNP as a thyroid hormone-disrupting chemical.

Birds and flame retardants: A review of the toxic effects on birds of historical and novel flame retardants

Flame retardants (FRs) are a diverse group of chemicals, many of which persist in the environment and bioaccumulate in biota. Although some FRs have been withdrawn from manufacturing and commerce (e.g., legacy FRs), many continue to be detected in the environment; moreover, their replacements and/or other novel FRs are also detected in biota. Here, we review and summarize the literature on the toxic effects of various FRs on birds. Birds integrate chemical information (exposure, effects) across space and time, making them ideal sentinels of environmental contamination. Following an adverse outcome pathway (AOP) approach, we synthesized information on 8 of the most commonly reported endpoints in avian FR toxicity research: molecular measures, thyroid-related measures, steroids, retinol, brain anatomy, behaviour, growth and development, and reproduction. We then identified which of these endpoints appear more/most sensitive to FR exposure, as determined by the frequency of significant effects across avian studies. The avian thyroid system, largely characterized by inconsistent changes in circulating thyroid hormones that were the only measure in many such studies, appears to be moderately sensitive to FR exposure relative to the other endpoints; circulating thyroid hormones, after reproductive measures, being the most frequently examined endpoint. A more comprehensive examination with concurrent measurements of multiple thyroid endpoints (e.g., thyroid gland, deiodinase enzymes) is recommended for future studies to more fully understand potential avian thyroid toxicity of FRs. More research is required to determine the effects of various FRs on avian retinol concentrations, inconsistently sensitive across species, and to concurrently assess multiple steroid hormones. Behaviour related to courtship and reproduction was the most sensitive of all selected endpoints, with significant effects recorded in every study. Among domesticated species (Galliformes), raptors (Accipitriformes and Falconiformes), songbirds (Passeriformes), and other species of birds (e.g. gulls), raptors seem to be the most sensitive to FR exposure across these measurements. We recommend that future avian research connect biochemical disruptions and changes in the brain to ecologically relevant endpoints, such as behaviour and reproduction. Moreover, connecting in vivo endpoints with molecular endpoints for non-domesticated avian species is also highly important, and essential to linking FR exposure with reduced fitness and population-level effects.

Permethrin is a potential thyroid-disrupting chemical: In vivo and in silico envidence

Permethrin (PM), one of the most heavily used synthetic pyrethroids, has the potential to interfere with thyroid hormones in mammals, however, the effect is poorly recognized in aquatic organisms. Herein, embryonic zebrafish were exposed to PM (0, 1, 3 and 10μg/L) until 72h post-fertilization. We demonstrated that PM readily accumulated in larvae with a preference for cis-PM, inhibited development and increased thyroxine and 3,5,3'-triiodothyronine levels accompanying increase in the transcription of most target genes, i.e., thyroid-stimulating hormone β, deiodinases, thyroid receptors, involved in the hypothalamic-pituitary-thyroid axis. Further Western blot analysis indicated that transthyretin (TTR) protein was significantly increased. Molecular docking analysis and molecular dynamics simulations revealed that PM fits into three hydrophobic binding pocket of TTR, one of the molecular targets of thyroid hormone disrupting chemicals (THDCs), and forms strong van der Waals interactions with six resides of TTR, including Leu8, Leu 101, Leu125, Thr214, Leu218 and Val229, thus altering TTR activity. Both in vivo and in silico studies clearly disclosed that PM potentially disrupts the thyroid endocrine system in fish. This study provides a rapid and cost-effective approach for identifying THDCs and the underlying mechanisms.

Acute exposure to synthetic pyrethroids causes bioconcentration and disruption of the hypothalamus-pituitary-thyroid axis in zebrafish embryos

Synthetic pyrethroids (SPs) have the potential to disrupt the thyroid endocrine system in mammals; however, little is known of the effects of SPs and underlying mechanisms in fish. In the current study, embryonic zebrafish were exposed to various concentrations (1, 3 and 10 μg/L) of bifenthrin (BF) or λ-cyhalothrin (λ-CH) until 72 h post fertilization, and body condition, bioaccumulation, thyroid hormone levels and transcription of related genes along the hypothalamus-pituitary-thyroid (HPT) axis examined. Body weight was significantly decreased in the λ-CH exposure groups, but not the BF exposure groups. BF and λ-CH markedly accumulated in the larvae, with concentrations ranging from 90.7 to 596.8 ng/g. In both exposure groups, alterations were observed in thyroxine (T4) and triiodothyronine (T3) levels. In addition, the majority of the HPT axis-related genes examined, including CRH, TSHβ, TTR, UGT1ab, Pax8, Dio2 and TRα, were significantly upregulated in the presence of BF. Compared to BF, λ-CH induced different transcriptional regulation patterns of the tested genes, in particular, significant stimulation of TTR, Pax8, Dio2 and TRα levels along with concomitant downregulation of Dio1. Molecular docking analyses revealed that at the atomic level, BF binds to thyroid hormone receptor (TRα) protein more potently than λ-CH, consequently affecting HPT axis signal transduction. In vitro and in silico experiments disclosed that during the early stages of zebrafish development, BF and λ-CH have the potential to disrupt thyroid endocrine system.

Effect of acetochlor on transcription of genes associated with oxidative stress, apoptosis, immunotoxicity and endocrine disruption in the early life stage of zebrafish

The study presented here aimed to characterize the effects of acetochlor on expression of genes related to endocrine disruption, oxidative stress, apoptosis and immune system in zebrafish during its embryo development. Different trends in gene expression were observed after exposure to 50, 100, 200μg/L acetochlor for 96h. Results demonstrated that the transcription patterns of many key genes involved in the hypothalamic-pituitary-gonadal/thyroid (HPG/HPT) axis (e.g., VTG1, ERβ1, CYP19a and TRα), cell apoptosis pathway (e.g., Bcl2, Bax, P53 and Cas8), as well as innate immunity (e.g., CXCL-C1C, IL-1β and TNFα) were affected in newly hatched zebrafish after exposure to acetochlor. In addition, the up-regulation of CAT, GPX, GPX1a, Cu/Zn-SOD and Ogg1 suggested acetochlor might trigger oxidative stress in zebrafish. These finding indicated that acetochlor could simultaneously induce multiple responses during zebrafish embryonic development, and bidirectional interactions among oxidative stress, apoptosis pathway, immune and endocrine systems might be present.

Biochemical and Transcriptomic Effects of Herring Gull Egg Extracts from Variably Contaminated Colonies of the Laurentian Great Lakes in Chicken Hepatocytes

Determining the effects of complex mixtures of environmental contaminants poses many challenges within the field of ecotoxicology. In this study, graded concentrations of herring gull egg extracts, collected from five Great Lakes breeding colonies with variable burdens of organohalogen contaminants (OHCs), were administered to chicken embryonic hepatocytes to determine effects on 7-ethoxyresorufin-O-deethylase (EROD) activity, porphyrin accumulation, and mRNA expression. EROD activity and porphyrin accumulation permitted the ranking of colonies based on the efficacy of eliciting an aryl hydrocarbon receptor-mediated response. An avian ToxChip polymerase chain reaction (PCR) array provided more exhaustive coverage in terms of potential toxicity pathways being affected, including xenobiotic and lipid metabolism and the thyroid hormone pathway. Herring gull eggs from Channel Shelter Island (CHSH, Lake Huron) and Gull Island (GULL, Lake Michigan) had among the highest OHC burdens, and extracts elicited a biochemical and transcriptomic response greater than that of extracts from the other three, less polluted colonies. For example, EROD EC50 values and porphyrin ECthreshold values were lower for CHSH and GULL extracts than for the other colonies. Extracts from CHSH and GULL altered 15 and 13 of 27 genes on the PCR array compared to no more than eight genes for the less contaminated sites. The combination of a well-established avian in vitro assay, two well-characterized biochemical assays, and the avian ToxChip PCR array permitted the geographical discrimination of variably contaminated herring gull eggs from the Great Lakes. Such high-throughput assays show potential promise as cost-effective tools for determining toxic potencies of complex mixtures in the environment.

A review on the effects of PBDEs on thyroid and reproduction systems in fish

The objective of this review was to summarize and discuss the effects of Polybrominated diphenyl ethers (PBDEs) on thyroid and reproduction systems in fish. We reviewed the evidences and mechanisms for PBDEs-induced thyroid and reproduction disruption, as well as the cross-talk between the two systems in fish. In thyroid disruption, we mainly paid attention to the effects of PBDEs on hypothalamic-pituitary-thyroid (HPT) axis, thyroid hormones (THs) transport and metabolism, thyroid receptors (TRs) and thyroid follicle histology. In reproduction disruption, we focused on the effects of PBDEs on steroid hormone production, expression of genes involved in steroidogenesis, and gonadal development. Despite that there is an interaction between thyroid and reproductive systems in fish, it is still remains unclear that PBDE-induced reproductive impairments are caused by direct effects on hypothalamic-pituitary-gonadal (HPG) functioning or by indirect action through cross-talk between the two systems. Future studies are needed to explore the relationships between reproductive toxicity and thyroid system disruption after PBDEs exposure.

Multiple biomarkers of the cytotoxicity induced by BDE-47 in human embryonic kidney cells

Polybrominated diphenyl ethers (PBDEs) are widely used as brominated flame-retardants in a variety of industrial products. Among these PBDEs, 2,2',4,4'-tetra-bromodiphenyl ether (BDE-47) is one of the most predominant congeners inducing multiple toxicities, including hepatotoxicity, neurotoxicity, cytotoxicity, genotoxicity, carcinogenecity and immunotoxicity in human body. In this study, the cytotoxicity of BDE-47 in human embryonic kidney cells (HEK293) was investigated by a set of bioassays, including cell proliferation, apoptosis, oxidative stress and metabolic responses as well as gene expressions related to apoptosis. Results showed that BDE-47 induced an inverted U-shaped curve of cell proliferation in HEK293 cells from 10(-6) to 10(-4) M. Cell apoptosis and ROS overproduction were detected at 10(-5) M of BDE-47 (p<0.05). In addition, the expressions of Bcl-2 family-encoding genes (Bad, Hrk and Bcl-2) increased significantly in 10(-4)M group (p<0.05). Metabolic responses indicated that BDE-47 mainly caused disturbance in energy metabolism marked by differentially altered ethanol, glutathione, creatine, aspartate, UDP-glucose and NAD(+). The increased lactate/alanine ratios indicated the higher reductive state induced by BDE-47 in all exposures confirmed by the overproduction of ROS.

Mycoplasma gallisepticum in vivo induced antigens expressed during infection in chickens

Until now only a few genes encoding virulence factors have been characterized in the avian pathogen Mycoplasma gallisepticum. In order to identify candidate targets associated with infection we applied an immunoscreening technique-in vivo induced antigen technology (IVIAT)-to detect immunogens of M. gallisepticum strain Rlow expressed preferentially during in vivo infection. We identified 13 in vivo-induced (IVI) proteins that correspond to different functional categories including: previously reported putative virulence factors (GapA, PlpA, Hlp3, VlhA 1.07 and VlhA 4.01), transport (PotE, MGA_0241 and 0654), translation (L2, L23, ValS), chaperone (GroEL) and a protein with unknown function (MGA_0042). To validate the in vivo antigenic reactivity, 10 IVI proteins were tested by Western blot analysis using serum samples collected from chickens experimentally (with strain Rlow) and naturally (outbreaks, N=3) infected with M. gallisepticum. All IVI proteins tested were immunogenic. To corroborate these results, we tested expression of IVI genes in chickens experimentally infected with M. gallisepticum Rlow, and in MRC-5 human lung fibroblasts cell culture by using relative real time reverse-transcription PCR (RT-PCR). With the exception of MGA_0338, all six genes tested (MGA_1199, 0042, 0654, 0712, 0928 and 0241) were upregulated at least at one time point during experimental infection (2-4 week post-infection). In contrast, the expression of seven out of eight IVI genes (MGA_1199, 0152, 0338, 0042, 0654, 0712, 0928) were downregulated in MRC-5 cell culture at both 2 and 4h PI; MGA_0241 was upregulated 2h PI. Our data suggest that the identified IVI antigens may have important roles in the pathogenesis of M. gallisepticum infection in vivo.

Impact of co-exposure with lead and decabromodiphenyl ether (BDE-209) on thyroid function in zebrafish larvae

Polybrominated diphenyl ethers (PBDEs) and metals are the main contaminants at waste electrical and electronic equipment ("e-waste") recycling sites. However, the potential environmental health effects of mixtures of PBDEs and metals are not known. We investigated co-exposure of lead (Pb) with decabromodiphenyl ether (BDE-209) on thyroid function in zebrafish larvae. Seven groups of embryos/larvae of zebrafish were treated with Pb (0, 2, 5, 10, 15, 20, and 30 μg/L), six groups were exposed to BDE-209 (0, 50, 100, 200, 400, and 800 μg/L), and nine groups of zebrafish larvae were treated with Pb and BDE-209 (5, 10, and 20 μg/L Pb; 50, 100, and 200 μg/L BDE-209). Embryos/larvae were exposed from 2h post-fertilization (hpf) until 144 hpf, and thyroid hormone (TH) content measured. Pb exposure significantly decreased whole-body TH contents (triiodothyroxine (T3) and thyroxine (T4)) but BDE-209 exposure significantly increased T3 and T4 levels. Pb or BDE-209 treatment alone caused a predicted downregulation of TH transport (i.e., expression of the mRNA or proteins of transthyretin). Chemical analyses showed Pb uptake to be increased by BDE-209, but BDE-209 bioconcentration was decreased and the ability to metabolize BDE-209 was reduced in the presence of Pb. We also found that a mixture of the two chemicals had a synergistic effect on TH levels in zebrafish.

A mechanistic view of polybrominated diphenyl ether (PBDE) developmental neurotoxicity

Polybrominated diphenyl ethers (PBDEs), extensively used in the past few decades as flame retardants in a variety of consumer products, have become world-wide persistent environmental pollutants. Levels in North America are usually higher than those in Europe and Asia, and body burden is 3-to-9-fold higher in infants and toddlers than in adults. The latter has raised concern for potential developmental toxicity and neurotoxicity of PBDEs. Experimental studies in animals and epidemiological observations in humans suggest that PBDEs may be developmental neurotoxicants. Pre- and/or post-natal exposure to PBDEs may cause long-lasting behavioral abnormalities, particularly in the domains of motor activity and cognition. The mechanisms underlying the developmental neurotoxic effects of PBDEs are not known, though several hypotheses have been put forward. One general mode of action relates to the ability of PBDEs to impair thyroid hormone homeostasis, thus indirectly affecting the developing brain. An alternative or additional mode of action involves a direct effect of PBDEs on nervous system cells; PBDEs can cause oxidative stress-related damage (DNA damage, mitochondrial dysfunction, apoptosis), and interfere with signal transduction (particularly calcium signaling), and with neurotransmitter systems. Important issues such as bioavailability and metabolism of PBDEs, extrapolation of results to low level of exposures, and the potential effects of interactions among PBDE congeners and between PBDEs and other contaminants also need to be taken into account.

Bioconcentration and metabolism of decabromodiphenyl ether (BDE-209) result in thyroid endocrine disruption in zebrafish larvae

Polybrominated diphenyl ethers (PBDEs) have the potential to disturb the thyroid endocrine system, but little is known of such effects or underlying mechanisms of BDE-209 in fish. In the present study, bioconcentration and metabolism of BDE-209 were investigated in zebrafish embryos exposed at concentrations of 0, 0.08, 0.38 and 1.92 mg/L in water until 14 days post-fertilization (dpf). Chemical analysis revealed that BDE-209 was accumulated in zebrafish larvae, while also metabolic products were detected, including octa- and nona-BDEs, with nona-BDEs being predominant. The exposure resulted in alterations of both triiodothyronine (T3) and thyroxine (T4) levels, indicating thyroid endocrine disruption. Gene transcription in the hypothalamic-pituitary-thyroid (HPT) axis was further examined, and the results showed that the genes encoding corticotrophin-releasing hormone (CRH) and thyroid-stimulating hormone (TSHβ) were transcriptionally significantly up-regulated. Genes involved in thyroid development (Pax8 and Nkx2.1) and synthesis (sodium/iodide symporter, NIS, thyroglobulin, TG) were also transcriptionally up-regulated. Up-regulation of mRNA for thyronine deiodinase (Dio1 and Dio2) and thyroid hormone receptors (TRα and TRβ) was also observed. However, the genes encoding proteins involved in TH transport (transthyretin, TTR) and metabolism (uridinediphosphate-glucuronosyl-transferase, UGT1ab) were transcriptionally significantly down-regulated. Furthermore, protein synthesis of TG was significantly up-regulated, while that of TTR was significantly reduced. These results suggest that the hypothalamic-pituitary-thyroid axis can be evaluated to determine thyroid endocrine disruption by BDE-209 in developing zebrafish larvae.

Effects of tris(1,3-dichloro-2-propyl) phosphate and tris(1-chloropropyl) phosphate on cytotoxicity and mRNA expression in primary cultures of avian hepatocytes and neuronal cells

Tris(1,3-dichloro-2-propyl) phosphate (TDCPP) and tris(1-chloropropyl) phosphate (TCPP) belong to a group of chemicals collectively known as triester organophosphate flame retardants (OPFRs). OPFRs are used in a wide range of consumer products and have been detected in biota, including free-living avian species; however, data on toxicological and molecular effects of exposure are limited. An in vitro screening approach was used to compare concentration-dependent effects of TDCPP and TCPP on cytotoxicity and messenger RNA (mRNA) expression in cultured hepatocytes and neuronal cells derived from embryonic chickens. TDCPP was toxic to hepatocytes (LC₅₀ = 60.3 ± 45.8μM) and neuronal cells (LC₅₀ = 28.7 ± 19.1μM), whereas TCPP did not affect viability in either cell type up to the highest concentration administered, 300μM. Real-time reverse transcription-PCR revealed alterations in mRNA abundance of genes associated with phase I and II metabolism, the thyroid hormone (TH) pathway, lipid regulation, and growth in hepatocytes. None of the transcripts measured in neuronal cells (D2, D3, RC3, and Oct-1) varied in response to TDCPP or TCPP exposure. Exposure to ≥ 10μM TDCPP and TCPP resulted in significant upregulation of CYP2H1 (4- to 8-fold), CYP3A37 (13- to 127-fold), and UGT1A9 (3.5- to 7-fold) mRNA levels. Transthyretin was significantly downregulated more than twofold by TCPP at 100μM; however, TDCPP did not alter its expression. Liver fatty acid-binding protein, TH-responsive spot 14-α, and insulin-like growth factor-1 were all downregulated (up to 10-fold) in hepatocytes exposed to ≥ 0.01μM TDCPP and TCPP. Taken together, our results indicate that genes associated with xenobiotic metabolism, the TH pathway, lipid regulation, and growth are vulnerable to TDCPP and TCPP administration in cultured avian hepatocytes. The mRNA expression data were similar to those from a previous study with hexabromocyclododecane.

Effects of perfluoroalkyl compounds on mRNA expression levels of thyroid hormone-responsive genes in primary cultures of avian neuronal cells

There is growing interest in assessing the neurotoxic and endocrine disrupting potential of perfluoroalkyl compounds (PFCs). Several studies have reported in vitro and in vivo effects related to neuronal development, neural cell differentiation, prenatal and postnatal development and behavior. PFC exposure altered hormone levels and the expression of hormone-responsive genes in mammalian and aquatic species. This study is the first to assess the effects of PFCs on messenger RNA (mRNA) expression in primary cultures of neuronal cells in two avian species: the domestic chicken (Gallus domesticus) and herring gull (Larus argentatus). The following thyroid hormone (TH)–responsive genes were examined using real-time reverse transcription-PCR: type II iodothyronine 5′-deiodinase (D2), D3, transthyretin (TTR), neurogranin (RC3), octamer motif–binding factor (Oct-1), and myelin basic protein. Several PFCs altered the mRNA expression levels of genes associated with the TH pathway in avian neuronal cells. Short-chained PFCs (less than eight carbons) altered the expression of TH-responsive genes (D2, D3, TTR, and RC3) in chicken embryonic neuronal cells to a greater extent than long-chained PFCs (more than or equal to eight carbons). Variable transcriptional changes were observed in herring gull embryonic neuronal cells exposed to short-chained PFCs; mRNA levels of Oct-1 and RC3 were upregulated. This is the first study to report that PFC exposure alters mRNA expression in primary cultures of avian neuronal cells and may provide insight into the possible mechanisms of action of PFCs in the avian brain.

Exposure to DE-71 alters thyroid hormone levels and gene transcription in the hypothalamic-pituitary-thyroid axis of zebrafish larvae

Polybrominated diphenyl ethers (PBDEs) have the potential to disrupt thyroid hormone homeostasis, but the molecular mechanisms underlying this process have not yet been clarified. In the present study, zebrafish (Danio rerio) embryos were exposed to a low concentration (0, 1, 3, and 10microg/L) of DE-71 from fertilization to 14 days thereafter. The whole-body content of thyroid hormone and transcription of genes in the hypothalamic-pituitary-thyroid (HPT) axis were analyzed. Exposure to up to 10microg/L of DE-71 significantly reduced thyroxine (T4) levels and significantly upregulated the transcription of corticotrophin-releasing hormone (CRH) and thyroid-stimulating hormone (TSHbeta) genes in a concentration-dependent manner. The transcription of genes involved in the synthesis of TH proteins, sodium/iodide symporter (Slc5a5), and thyroglobulin (TG) and the transcription of marker genes associated with early thyroid development (Pax8 and Nkx2.1) were significantly upregulated upon DE-71 exposure. The expression of thyronine deiodinase (Deio1 and Deio2) mRNAs was also significantly upregulated, possibly as a compensatory response to the decreased T4 levels. However, DE-71 exposure resulted in the downregulation of transthyretin (TTR) gene transcription and did not affect the transcription of thyroid hormone receptors (TRs). Exposure to DE-71 significantly induced the transcription of the uridinediphosphate-glucuronosyltransferase (UGT1ab) gene. The results of our study confirmed the reliability of the zebrafish larvae as models for assessment of the developmental toxicity of PBDEs and transcription of genes of the HPT axis can evaluate the potential mechanisms of thyroid disruption.

Chicken embryo cardiomyocyte cultures--a new approach for studying effects of halogenated aromatic hydrocarbons in the avian heart

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), polychlorinated biphenyls (PCBs), and other halogenated aromatic hydrocarbons (HAHs) elicit a variety of adverse biological effects on the cardiovascular systems of mammalian, piscine and avian species. Many of the cardiotoxic effects of HAHs are mediated by the aryl hydrocarbon receptor (AHR). Induction of cytochrome P4501A (CYP1A) is a well-known AHR-dependent response to HAHs in the liver, but there are a limited number of studies on CYP1A induction by these compounds in the heart. We used an in vitro approach to examine effects of TCDD and 3,3',4,4'-tetrachlorobiphenyl (PCB 77) on CYP1A in the avian heart. The responses of primary cultures of chicken embryo cardiomyocytes (CEC) and chicken embryo hepatocytes (CEH) to TCDD and PCB 77 were compared using immunofluorescence staining for CYP1A, the ethoxyresorufin-O-deethylase (EROD) assay, and real-time RT-PCR analysis of CYP1A4 mRNA and CYP1A5 mRNA. Immunofluorescent detection of CYP1A indicated that induction of CYP1A by TCDD was localized within the cytoplasm of CEC cells. EROD activity and CYP1A4/5 mRNA levels were strongly induced in CEC and CEH cultures by TCDD and PCB 77, and the shapes of the concentration-response curves in CEC and CEH cultures were similar. The studies provide clear evidence that the AHR signaling pathway is induced by TCDD and PCB 77 in CEC, and establish a new in vitro approach for studying the effects of HAHs in the avian heart. Induction of CYP1A5 by TCDD in avian cardiomyocytes is a novel finding, and might help direct future studies on mechanisms of action of HAHs in the heart.

Detection of polybrominated diphenyl ethers in herring gull (Larus argentatus) brains: effects on mRNA expression in cultured neuronal cells

In recent years, polybrominated diphenyl ethers (PBDEs) have been detected at increasing levels in the environment due to their widespread use as flame retardants. PBDEs can affect thyroid hormone homeostasis and the cholinergic neurotransmitter system. In this study, several PBDE congeners were detected in whole brain samples and neuronal cells of herring gulls (Larus argentatus). A herring gull neuronal cell culture method was used to determine the effects of PBDEs on cytotoxicity and mRNA expression. Real-time RT-PCR assays were developed for genes associated with the thyroid hormone pathway (thyroid hormone receptors [TR alpha and beta], transthyretin [TTR]), and the cholinergic system (neuronal nicotinic acetylcholine receptor alpha-7 [nAChR alpha-7]). Administration of T3 resulted in a significant up-regulation of the two TRs and a significant down-regulation of TTR. TTR was also down-regulated by the commercial penta-BDE mixture, DE-71. In contrast, neither DE-71, nor BDE-47, -99, or -100 altered the mRNA levels of the TRs or nAChR alpha-7. The in vitro approach was a relevant model system for assessing the effects of PBDEs on cytotoxicity and mRNA expression. Herring gull neuronal cells were responsive to both T3 and PBDEs although, receptors associated with two predicted mechanisms of PBDE action were not effective molecular biomarkers of exposure.

Effects of hexabromocyclododecane and polybrominated diphenyl ethers on mRNA expression in chicken (Gallus domesticus) hepatocytes

Hexabromocyclododecane (HBCD) and polybrominated diphenyl ethers (PBDEs) are additive flame retardants used in a wide range of consumer products. Both compounds have been detected in free-living avian species, but toxicological and molecular end points of exposure are limited. An in vitro approach was used to compare concentration-dependent effects of HBCD and the commercial penta-brominated diphenyl ether mixture DE-71 on cytotoxicity and mRNA expression in cultured hepatocytes derived from embryonic chickens. Neither HBCD-alpha, HBCD-technical mixture (TM), nor DE-71 effected hepatocyte viability at the highest concentrations assessed (30-100 microM). Real-time RT-PCR assays were developed to quantify changes in mRNA abundance of genes associated with chicken xenobiotic-sensing orphan nuclear receptor activation, the thyroid hormone (TH) pathway, and lipid regulation. Exposure to >or= 1 microM HBCD-alpha and HBCD-TM resulted in significant upregulation of cytochrome P450 (CYP) 2H1 (fourfold to sevenfold) and CYP3A37 (5- to 30-fold) at 24 and 36 h. In contrast, 30 microM DE-71 caused a twofold increase of CYP2H1 only. UGT1A9 expression was only upregulated by HBCD-alpha to a maximum of fourfold at >or= 1 microM. Transthyretin, thyroid hormone-responsive spot 14-alpha, and liver fatty acid-binding protein were all significantly downregulated (up to sevenfold) for cells exposed to >or= 1 microM HBCD-alpha and HBCD-TM. DE-71 also downregulated these three target genes twofold to fivefold at concentrations >or= 3 microM. Taken together, our results indicate that xenobiotic-metabolizing enzymes and genes associated with the TH pathway and lipid regulation are vulnerable to HBCD and DE-71 administration in cultured avian hepatocytes and might be useful molecular markers of exposure.